[llvm] [AMDGPU] Unify handling of BITOP3 operation (PR #132019)

[Jakub Chlanda via llvm-commits]

jchlanda wrote:

@arsenm are you OK with this going in?

I appreciate it's hard to avoid duplication supporting both SelectionDAG and GlobalIsel, but thought that having (almost) identical helpers was where I would draw a line.

This was triggered by our downstream static analysis tool picking up on potentially uninitialized values ([here](https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.cpp#L3878) and [here](https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp#L3805)), so alternatively, I could provide a patch to address that.

https://github.com/llvm/llvm-project/pull/132019